If you’ve ever wondered what different switch mode power supplies would sound like if used to drive a speaker, here is an untested circuit you might try.  Using a zener clamped piezoelectric transducer as a contact microphone to push/pull the feedback inputs on a pair of switch modes should modulate the output of the power supplies inversely.

Where it gets interesting is the unique effect that the feedback control mechanism will have.  Switchmodes employ a variety of techniques in the feedback loop to improve response and mitigate instability.  Often the datasheet will mention PID, pulse skipping, pulse frequency modulation and other interesting control dynamics.  The effect might be more pronounced with older controllers that use a lower switching frequency.

The important consideration here is that the switch modes are bidirectional which limits the application to Split-pi,  Cuk topology, or any topology with active/synchronous rectification.  If you happen to give it a try, we’d love to hear [about] it!